C-calling convention PLUS balancing stack

[Ksbunker]

I have a function "FormatString()" with in theory an unlimited number of parameters, however it is called using the c-calling convention so the stack has to be manually balanced after call.

In the example below, the function takes 3 parameters (therefore 3h*4h=0Ch) and the stack requires "add esp, 0Ch"

Is there any means of automating the stack balancing after the Call. Using macro's comes to mind, but im not sure of their exact usage in this situation.

I did read that the INVOKE macro did this, but im not 100% sure.

Any help appreciated.

Code Select Expand

.data
  szFormat BYTE "%s %s", 0
  sz1 BYTE "String1", 0
  sz2 BYTE "String2", 0
.code
  push OFFSET sz2
  push OFFSET sz1
  push OFFSET szFormat
  call FormatString
  add esp, 0Ch

[sinsi]

Just prototype it before you use it -

Code Select Expand

FormatString PROTO C :VARARG
Then using invoke will clean the stack up for you.

[Ksbunker]

Many thanks and fast reply!

[MichaelW]

I see that sinsi provided the answer, but since I bothered to code this I'll post it anyway.

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; «««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««
    include \masm32\include\masm32rt.inc
; «««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««
    .data
    .code
; «««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««
varargs proc C p:DWORD, v:VARARG
    ret
varargs endp
; «««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««
start:
; «««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««
    invoke varargs, 1, 2
    print uhex$(esp),13,10
    invoke varargs, 1, 2, 3
    print uhex$(esp),13,10
    invoke varargs, 1, 2, 3, 4
    print uhex$(esp),13,10
    invoke crt_printf, chr$("%d %d %c"), 1, 2, 10
    print uhex$(esp),13,10,13,10

    inkey "Press any key to exit..."
    exit
; «««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««
end start


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00401005 6A02                   push    2
00401007 6A01                   push    1
00401009 E8F2FFFFFF             call    fn_00401000
0040100E 83C408                 add     esp,8
. . .
00401030 6A03                   push    3
00401032 6A02                   push    2
00401034 6A01                   push    1
00401036 E8C5FFFFFF             call    fn_00401000
0040103B 83C40C                 add     esp,0Ch
. . .


[sinsi]

I wasn't sure whether to do "proto c :dword, :vararg" so I cheated and looked at wsprintf in user.inc  :bdg

[mitchi]

I think that this is the best calling convention for 32 bit Windows  :U

[Jimg]

I like syscall better.  I avoid C as much as possible :bg

[mitchi]

syscall? That's on Linux no?

[Jimg]

Maybe, but it's also in masm.  It doesn't prepend a worthless underline character.

[mitchi]

Ah, sorry I was confused by another meaning of syscall I know.
Yes, syscall convention is good! ;)

[Vortex]

The syscall calling convention is useful to define symbols with no decoration.